1. The Field of the Invention
The present invention relates to inflatable safety restraint systems for vehicles. More specifically, the present invention relates to a novel apparatus and method for improving leak detection in an inflator for an airbag assembly.
2. The Relevant Technology
The inclusion of inflatable safety restraint devices, or airbags, is now a legal requirement for many new vehicles. Airbags are typically installed in the steering wheel and in the dashboard on the passenger side of a car. In the event of an accident, an accelerometer within the vehicle measures the abnormal deceleration and triggers the release of pressurized gases. The expanding gases fill the airbags, which immediately inflate in front of the driver and passenger to protect them from impact against the steering and dashboard components. Side impact airbags have also been developed in response to the need for similar protection from impacts in a lateral direction, or against the side of the vehicle.
The pressurized gas typically originates within a pressure vessel called an xe2x80x9cinflator.xe2x80x9d Inflators provide the pressurized gas in many different ways. Some inflators, termed xe2x80x9cstored gas inflators,xe2x80x9d simply store the gas in a high-pressure state, and open to release the gas during impact. xe2x80x9cPyrotechnicxe2x80x9d inflators, by contrast, do not store gas; rather, they contain generants that, upon ignition, react to produce the gas. xe2x80x9cHybridxe2x80x9d inflators utilize compressed gas in combination with pyrotechnics to produce the inflation gas. In some instances, the pyrotechnic can also serve to open the inflator to permit the gases to escape.
Each type of inflator must generally be sealed. In the case of compressed gas inflators, it is necessary to keep the compressed gas from escaping. For pyrotechnic and hybrid inflators, the generants must typically be sealed off from ambient air to avoid degradation from moisture and other contaminants. Inflators utilizing solid generants are typically activated by an initiator, which converts an electric impulse to heat in order to ignite the charge.
Such initiators often have electrical contacts, or prongs, protruding from the inflator to receive the electric impulse from wiring or a socket within the vehicle. Consequently, an opening must be provided in the wall of the inflator so that the prongs can extend outside the pressurized compartment(s) of the inflator. In order to ensure that the generant remains viable, a membrane, or pressure dome, may be positioned around the initiator to separate the generant from the initiator and whatever ambient air may be present in the vicinity of the initiator. The pressure dome is designed to disintegrate upon activation of the initiator, thereby permitting the heat of the initiator to reach the generant.
Upon disintegration of the pressure dome, the cavity is then exposed to the gases created by the reaction of the generant. Directly after ignition of the generant, these gases are hot and highly-pressurized, especially in the vicinity of the initiator, where the generant has reacted. If these gases exit the inflator through the opening at such an elevated temperature and pressure, they can potentially damage the vehicle or injure vehicle occupants. Consequently, it is desirable to encase the initiator in some type of insert that effectively plugs the opening, while still permitting passage of the initiator through the opening.
However, it is difficult to form a reliable seal between the inflator and the insert, and still more difficult to effectively test whether or not the insert has effectively sealed the opening. Thus, the initiator is located inside a cavity that may or may not be open to ambient air. Such an arrangement, in which there is a low-pressure cavity within the initiator, can cause a number of problems, particularly with leak detection.
Typically, inflators are checked for leaks prior to installation in a vehicle. Leak detection may be accomplished by, first, filling the inflator with the appropriate mixture of gases. Often, a small percentage of trace material, consisting of an easily detected gas, is added. For example, helium is often added to inflators because helium occurs only in trace amounts in nature, and has a unique atomic weight that is easily detectable through mass spectrometry or other known methods. Radioactive isotopes may also be effectively used for trace materials. The inflator is then placed in a testing chamber, and the testing chamber is evacuated and then sealed. After a certain period of time, the amount of the trace material within the chamber is measured and recorded. If more than a trace amount of the gas is detected, the inflator is rejected and typically scrapped.
However, when there is a cavity within the inflator, that may or may not be sealed from the testing chamber, it is difficult to recognize whether detected leaks are from the main pressurized internal compartment of the inflator, which must remain sealed, or from the cavity, for which sealing from ambient air is not critical. More specifically, xe2x80x9cvirtual leaksxe2x80x9d and xe2x80x9cmasked leaksxe2x80x9d may be caused by such a cavity.
A xe2x80x9cvirtual leakxe2x80x9d exists when gases remaining in the cavity during the evacuation of the testing chamber emerge after evacuation. Often the processes of assembling and filling the inflator leaves a certain amount of residual gas, including the trace material, within the cavity. Alternatively, these gases may be temporarily absorbed by the materials of the cavity, and may remain present until the cavity is subjected to the low pressure of the testing chamber.
Such a leak is a xe2x80x9cvirtual leakxe2x80x9d because there is no real leak in the main internal compartment of the inflator, but the gas sensing equipment registers the presence of the gases from the cavity. Since it is difficult to detect exactly which part of the inflator is the source of gases detected in the testing chamber without comprehensive and time-consuming tests, it is often assumed that the inflator is defective if any significant amount of the trace material is present in the chamber after evacuation. As a result, virtual leaks result in the scrapping of many perfectly usable inflators. The lower yield of the inflator production process causes inflators, and airbag systems in general, to be more expensive, and therefore less widely available as lifesaving devices.
A xe2x80x9cmasked leakxe2x80x9d occurs when there is an actual leak in the main internal compartment of the inflator, for example, in the pressure dome, but the leak is not detected. Gas leaks from the main internal compartment, which is at comparatively high pressure, into the cavity, which is at a lower pressure. However, the insert acts to keep the gases from escaping the cavity at a significant rate. Thus, after evacuation of the testing chamber, no significant amount of the trace material is registered.
Such a leak is potentially dangerous because a real leak exists in the inflator, and over the operating life of the inflator, which may be as much as 15 years, the compressed gas will leak out of the inflator. Without the compressed gas, it is likely that the airbag cushion will not inflate enough to effect occupants of the vehicle. It is also possible that the generant will become contaminated or moistened by exposure to ambient air. Thus, the generant may misfire, causing insufficient inflation of the cushion and potential danger to occupants of the vehicle. The leak is effectively xe2x80x9cmaskedxe2x80x9d because the insert does not permit the leaking gases to escape at a detectable rate. Even though the inflator is defective, it passes inspection and is installed in a vehicle.
Virtual leaks and masked leaks generally can be traced to the same root cause: the unpredictability of the seal provided by the insert. In the case of a virtual leak, the insert permits comparatively free flow of gases out of the cavity, and in the case of a masked leak, the insert seals off the cavity enough to prevent detection of the leak. Both problems are a result of the fact that the integrity (gastight sealing effectiveness) of the insert is unknown and subject to wide variation.
Unfortunately, known methods of allowing for fluid passage through an obstacle at a limited flow rate are generally unusable in the presence of gases at combustion temperatures and pressures. For example, if a comparatively large hole is provided in the insert, the result is that a xe2x80x9cblowby pathxe2x80x9d exists in the inflator. Rather than entering the cushion, as intended, the combustion and compressed gases are permitted to blow through the insert at a high flow rate. Thus, the cushion is not sufficiently inflated, and a dangerous buildup of hot, pressurized gas is created outside the inflator, within the vehicle.
If, instead of a large hole, one or more smaller holes are formed in the insert, a xe2x80x9cballistic leakxe2x80x9d is likely to occur. Ballistic leaking occurs when leaking gases are at a high enough temperature and pressure to erode the inflator material (typically metal), and even burn into the interior of the vehicle. Tight flow restrictions, such as small holes, multiply the speed of exiting gases so that a jet of hot, high pressure gas exits the hole.
Since erosion of a solid material by a fluid flow is generally proportional to the speed of the fluid, the rapidly travelling gases erode the walls of a small hole at a high rate of speed. As a result, hot pieces of the inflator insert material are entrained in the gas flow, thereby increasing the cutting force of the jet. The jet can cut through parts of the vehicle interior, or even cause a fire in the vehicle. Ballistic leaks are thus highly undesirable because they have the potential to damage the vehicle and injure occupants.
Consequently, there is a need, unfulfilled by the prior art, for an inflator insert that permits the passage of gas at a predictable rate. More specifically, the insert should preferably permit gases at low pressure to flow through the insert over time, while limiting the speed at which high pressure gases are able to escape. Preferably, the insert should not contain flow restrictions that tend to channel pressurized gases into a narrow passageway, so that ballistic leaking does not occur.
Additionally, such an insert should preferably be structurally strong, so that it can tolerate the stresses induced by installation within the inflator and operation of the inflator. Furthermore, the insert should preferably be inexpensive, easy to manufacture, and simple to install. Yet further, the insert may beneficially be made compatible with existing initiator and inflator designs.
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available inflators. Thus, it is an overall objective of the present invention to provide an apparatus and method for plugging an opening of an inflator, such that gas is permitted to flow through the insert at a predictable and controlled flow rate.
To achieve the foregoing objective, and in accordance with the invention as embodied and broadly described herein in the preferred embodiment, an inflator comprising a novel insert is provided. According to certain embodiments, the inflator contains compressed gas in a main internal compartment of the inflator; an outer wall of the inflator is configured as a pressure vessel to keep the compressed gas from exiting the main internal compartment.
The inflator may also contain a pyrotechnic assembly in which a measured quantity of generant, or pyrotechnic material, is stored. The pyrotechnic assembly may be seated within an opening in the outer wall of the inflator. Preferably, the pyrotechnic assembly provides a path from the pyrotechnic material to the outside of the inflator so that the pyrotechnic material can be ignited by an electrical impulse originating outside the inflator.
The pyrotechnic assembly may have a housing designed to contain the various components of the assembly. More specifically, the housing may have a larger compartment containing the pyrotechnic material. The larger compartment may separated from the main internal compartment by a cap seated within the housing. In certain embodiments, the cap need not provide a seal against the compressed gas, but must simply keep the pyrotechnic material from exiting the pyrotechnic assembly to enter the main internal compartment. However, the pyrotechnic material is preferably kept isolated from the ambient air outside the inflator by a membrane, or pressure dome, abutting the pyrotechnic material, opposite the cap.
The pressure dome effectively forms a cavity within the inflator; the cavity is kept separate from the compressed gases of the main internal compartment, and the pyrotechnic material, by the pressure dome. Thus, the cavity may be left open to ambient air without losing the compressed gas in the main internal compartment or subjecting the pyrotechnic material to contaminants. An initiator may then be housed in the cavity, with an ignition head positioned close to the pressure dome to ignite the pyrotechnic material through the pressure dome.
Upon ignition of the inflator, the pressure dome disintegrates and the hot gases enter the cavity at high speed. For the reasons described above, these gases are not allowed to flow past the initiator and out of the inflator through the opening. Rather, an insert is preferably provided to plug the bore of the housing without interfering with operation of the initiator. Thus, the insert may have an annular configuration designed to encircle the initiator. Preferably, the insert is made to relatively tightly engage the initiator, as well as the bore of the pyrotechnic assembly housing, to eliminate any blowby path.
As described above, it is somewhat difficult to form a reliable seal between the insert and the initiator, and between the insert and the bore of the pyrotechnic housing. Thus, in order to avoid the problems described above in connection with leak detection, the insert is preferably made to permit passage of gas through the insert, but only at a limited flow rate. Thus, both residual trace gases and gases leaking from the main internal compartment, through the pressure dome, may escape through the insert for detection. However, no effective blowby path exists because the reaction of the pyrotechnic material exhausts itself before a significant amount of gas is able to escape through the insert. In order to avoid ballistic leaking, the insert preferably operates without any small, concentrated passageways that may tend to create a pressurized gas jet exiting the inflator.
In order to provide such controlled passage of gases, the insert may, in certain configurations, have a porous construction. Porous materials typically have a grainy texture, with small interstices between the grains, through which gases can flow. However, no straight flow path exists so gas passage is impeded and slowed by the porous structure. As a result, no blowby path is formed. Additionally, no ballistic jet is able to form because all paths through the porous material are more or less equally impeded, so flows do not concentrate in a single path, and all gas flows are slowed by the grainy texture.
Such a porous insert may be provided in a variety of ways. For example, certain materials, such as woods, are naturally porous. However, the insert of the present invention is preferably able to withstand the stresses of installation and the impact stresses caused by the ignition of the pyrotechnic material. Thus, it may be desirable to use a structurally stronger material, such as a metal.
Since most metals have a naturally non-porous structure, the material chosen is preferably processed to provide such a texture. For example, a metal powder of a suitable consistency may be provided through the use of a plasma spray. The metal powder may then be compressed into the proper shape, for example, by applying high pressure at ambient temperature to press the powder into a mold to form a compacted mass. Then, the compacted mass may be sintered, or essentially baked in a high-temperature oven, to fuse the powder grains and create a solid structure. The resulting insert possesses a porous structure and yet has a high degree of structural strength.
The insert may be installed, and the inflator may be filled with compressed gas, if it is of a hybrid type. Alternatively, the inflator may simply be a pyrotechnic type in which no significant amount of compressed gas is stored. In either case, the trace gas may be added for detection purposes. Then, the inflator may be tested to determine whether or not the pressure dome effectively seals the main internal compartment from the ambient atmosphere.
Testing may be accomplished by first placing the inflator into a testing chamber. Then, the testing chamber may be substantially evacuated or brought to a very low pressure. Once the chamber obtains the desired pressure, the inflator may simply be allowed to sit in the low-pressure chamber for a period of time so that any residual gases in the cavity can bleed out through the insert. These gases may then be removed through further evacuation of the chamber. During the bleed-out process, the level of the trace gas within the testing chamber may be continually monitored, if desired.
After the bleed-out period has elapsed, the chamber may be sealed off so that all additional gases exiting the inflator remain in the chamber. The amount of the trace gas may then be detected and recorded. If more than a threshold quantity of the trace gas is detected, the inflator may then be set aside for reworking, scrapping, or discarding.
Through operation of the insert, the results of the leak detection test are made much more reliable because the residual gases from the cavity have already bled out through the insert, prior to detection. Thus, any trace gases detected after the bleed-out must originate in the main internal compartment of the inflator. Consequently, leaks in the pressure dome cannot be masked, and residual trace gases in the cavity are cleared out so that they are unable to cause a virtual leak, or false leak reading.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.